Such apparatus is, in particular, used in assistance on a vehicle, so as to make it possible, whenever necessary, to actuate the hydraulic motor as a supplementary or assistance motor, for moving the vehicle under difficult movement conditions, such as slippery ground, steep slope, etc. Typically, the hydraulic motor is arranged on an axle of the vehicle that is an axle on which the wheels are not driven wheels when the vehicle is in normal advance mode; thus, when the hydraulic transmission apparatus is activated, the vehicle has additional driven wheels.
A particularly important application of the invention relates to hydraulic assistance apparatus mounted on road vehicles such as heavy goods vehicles that can travel at some speed (over 50 kilometers per hour (km/h)). For such an application, the hydraulic assistance apparatus must satisfy certain additional constraints: firstly, it must guarantee full safety when the vehicle is traveling at high speeds, i.e. it must be almost impossible for the assistance motors to be triggered in untimely manner; secondly the apparatus must make it possible for the assistance motors to be switched on while the vehicle is advancing, and not merely when it is at a standstill or while it is traveling at very low speed (less than 5 km/h).
The accumulator is a hydraulic accumulator of the rechargeable spring or gas type that contains pressurized fluid and is capable of storing hydraulic energy in the form of hydrostatic energy and of delivering hydraulic energy using the stored hydrostatic energy.
In order to enable the hydraulic transmission apparatus to be relatively practical to use, it is necessary for it to be quite simple and quick to activate or to deactivate.
This constraint is particularly important when the assistance motor is a motor of the positively declutchable type that has radial pistons arranged to act on an undulating cam, it being possible for said radial pistons to be retracted into the cylinders in such manner as to enable the motor to be positively declutched. In such a motor, the need for rapid switch-over from unassisted mode to assisted mode and vice versa does not only concern the ergonomics of the apparatus; quick positive clutching/declutching of the motor is also necessary in order to avoid damaging the motor during positive clutching or declutching, by limiting any rattling and jolting that is caused by such operations.
In order to facilitate positive clutching of the motor, it is possible, in known manner, to use an accumulator. An accumulator constitutes a supplementary pressurized fluid source; the pressure that it delivers can be transmitted to the motor ducts in order to accelerate deployment of the pistons out of the cylinders, and thus in order to accelerate positive clutching.